The osmotic diuretics can be relied upon to reduce intracranial pressure in patients wth conditons such as brain tumors and cerebral trauma. The deficiencies in this mode of therapy are brief duration of action (3-4 hours) and lack of dehydration with repeated drug administration. Our goals are to understand the mechanisms of brain dehydration with osmotic diuretics so that optimal conditions can be used to increase the efficiency of these agents. To this end, we propose to determine the effect of rate of administration of the diuretic on the degree of brain dehydration. The effectiveness of glycerol, urea and mannitol will be compared. The dose response of these agents will be determined in rats using regional measurements of brain specific gravity to quantify brain dehydration. A correlation will be made between plasma osmolality and brain dehydration. These results will be used to help answer the question: Is it better to use high initial drug levels to produce prolonged dehydration of brain or is it better to use lower levels for longer periods of time?